In a general type of low pressure mercury vapor discharge lamp, such as a light-bulb-shaped fluorescent lamp comprising an outer housing accommodating a fluorescent tube whose inner surface is covered with a phosphor, the mercury vapor pressure in the tube is controlled at a proper value by using an amalgam in order to prevent deterioration of luminous efficiency at a high temperature. In this case, the luminous efficiency depends on the temperature of the amalgam. The luminous efficiency will deteriorate when the amalgam temperature exceeds the proper value. In other words, the problem with the general low pressure mercury vapor discharge lamp is how to control the upper limit of the amalgam temperature.
In order to solve the above problem, a low pressure mercury vapor discharge lamp with amalgam temperature control is suggested, and this is accomplished in the prior art by covering the outer surface of a slender pipe containing amalgam with a heat radiation auxiliary component of a resin whose heat conductivity is better than that of the air. A further preferable example is disclosed in Japanese Laid-Open Patent Application (Tokukai-Sho) No. 61-225753, which discloses a low pressure mercury vapor discharge lamp in which a heat radiation auxiliary component covering a slender pipe contacts a housing, and the heat of the slender pipe is dissipated through the cover to the open air side.
Such a conventional low pressure mercury vapor discharge lamp comprises a slender pipe containing an amalgam. The outer surface of the pipe is covered with a heat radiation auxiliary component of a resin whose heat conductivity is better than that of the air, and the heat radiation auxiliary component is contacted with a housing. There is not, however, a sufficient difference between the temperature of the space in the housing and of the housing's inner surface in contact with the heat radiation auxiliary component, and the temperature of the amalgam container at the steady lighting of the lamp. Therefore, the heat of the amalgam container is not fully radiated, an the overheating of the amalgam cannot be fully controlled.
As such, there remains an opportunity to improve the low pressure mercury vapor discharge lamp. In particular, there exists a need for a low pressure mercury vapor discharge lamp that controls the amalgam temperature at a proper value and improves the luminous efficiency.